Problems arise when fluids trapped in the casing/casing annulus of an oil or gas well expand when heated as a result of production of hot fluids from the producing horizon into the wellbore. This expansion results in buildup of pressure in the annulus if no effort is undertaken to vent or otherwise mitigate the pressure buildup. This situation is commonly referred to as “annular pressure buildup (APB),” and it can result in either collapse of the inner casing string or burst of the outer casing string. Either of these conditions (burst or collapse) could potentially compromise the mechanical integrity of the oil or gas well. Over the years, a number of methods have been developed to address APB.
Vacuum insulated tubing (VIT) has been utilized to limit the transfer of heat from the wellbore to the fluids in the trapped casing/casing annulus, thereby serving to prevent deleterious APB. See, e.g., Segreto, U.S. Pat. No. 7,207,603.
Some APB mitigation efforts have involved placement of a compressible fluid, such as nitrogen (N2), in the trapped annulus during the cement job to limit the pressure buildup associated with expansion of the trapped fluid. See, e.g., Williamson et al., U.S. Pat. No. 4,109,725. While such methods can help limit the pressure in the annulus by liquefying the compressible fluid, the resulting pressures can still be quite high.
Insulating fluid/gel has been placed in the tubing/casing annulus in an effort to limit the transfer of heat due to convection from the wellbore to the fluids in the trapped casing/casing annuls. Methods utilizing such insulating fluid/gel effect APB mitigation in a manner similar to those employing VIT. See, e.g., Lon et al., U.S. Pat. No. 4,877,542.
In some instances, APB mitigation efforts have involved strapping a compressible solid material, such as foam or hollow particles, to the outside of the inner casing string to accommodate expansion of the fluids in the annulus by effectively “increasing” the volume in the annulus as the solid material compresses. See, e.g., Vargo et al., U.S. Pat. No. 7,096,944.
Another strategy for mitigating APB is to place a fluid or other material in the annulus that will “shrink” when activated due to heat and/or time. See, e.g., Hermes et al., United States Patent Application Publication No. 20070114033 A1, wherein methyl methacrylate is so used.
Burst and/or collapse disks have been employed to act as a pressure relief means and to allow the heated fluid in the annulus to “vent” through the disc. See, e.g., Staudt, U.S. Pat. No. 6,457,528.
In yet another APB mitigation technique, one can drill a hole in the outer casing string and allow the fluids to vent through the hole or via a pressure relief device placed in the hole. See, e.g., Haugen et al., U.S. Pat. No. 4,732,211.
Despite the variety of APB mitigation techniques described above, APB remains a serious problem—particularly for subsea operations. Accordingly, methods and systems that can better/further mitigate APB, either by themselves or in concert with one or more of the above-described techniques, would be particularly beneficial—particularly wherein such methods and systems can mitigate APB in subsea operations, and especially in deepwater operations.